A story of hope as well as horror, “Angkor Awakens: A Portrait of Cambodia” revisits a nightmare that humanity shouldn’t forget and, at the same time, documents the recovery and increasing optimism of a nation associated with 1970s killing-fields terror.

Directed by Robert H. Lieberman (“They Call It Myanmar”) and featuring interviews, archival footage and shadow puppetry, this worthy documentary screening at the Clay Theatre begins with a journey through Cambodian history, from the Angkor empire to the present.

Lieberman focuses on the second half of the 1970s, when the war in neighboring Vietnam enabled the murderous Khmer Rouge to take power of Cambodia.

Seeking to create a classless society, the Khmer Rouge cleared out cities and forced people into the countryside. Executions, starvation, poor medical care, and other factors caused an estimated 2 million deaths.

Lieberman also covers the U.S. bombing of Cambodia, a show of Cold War might by Richard Nixon and Henry Kissinger. (In a clip from a Democratic presidential debate, we see candidate Bernie Sanders blasting that secret action in Southeast Asia.)

The interviewees consist of politicians (including current prime minister and strongman Hun Sen), scholars, journalists, artists, and, most compellingly, genocide survivors, who provide firsthand horror stories while their children and grandchildren demonstrate how war trauma reverberates through generations.

A woman notes that if she’d worn glasses during the Khmer Rouge era, she would have been perceived as an intellectual and killed.

A young boy begins crying when the conversation turns to how his mother suffered during that period.

The film closes, however, on an optimistic note, with young Cambodians expressing hope for the future of their country.

The commentary of these and other survivors and their descendants makes the sometimes dry documentary illuminating and often affecting.